Atmospheric influence on fatigue crack propagation in titanium alloys at elevated temperature

The fatigue crack propagation behavior of a Ti-6Al-4V alloy has been investigated at room temperature and at 300°C. Tests were run in air, high vacuum, and some other environments with controlled partial pressure of watervapor and oxygen. The enhancement of the fatigue crack growth rates observed in air in comparison to high vacuum, considered as an inert environment, is clearly attributed to the presence of water vapor. Tests in a controlled environment demonstrate that very low partial pressure can accelerate crack propagation. On the basis of previous studies on Al alloys and steels, two controlling mechanisms are considered and discussed, namely, a propagation-assisted water vapor adsorption and a hydrogen-assisted propagation.